Adaptive optical signal processing with multimode waveguides

ABSTRACT

Optical signals are passed in an optical medium using an approach that facilitates the mitigation of interference. According to an example embodiment, a filtering-type approach is used with an optical signal conveyed in an optical fiber, such as a multimode fiber (MMF) or a multimode waveguide. Adaptive spatial domain signal processing, responsive to a feedback signal indicative of data conveyed in the multimode waveguide, is used to mitigate interference in optical signals conveyed in the multimode waveguide.

RELATED PATENT DOCUMENTS

This patent document is a continuation under 35 U.S.C. § 120 of U.S.patent application Ser. No. 11/940,199 filed on Nov. 14, 2007 (U.S. Pat.No. 7,509,002); which is a continuation of U.S. patent application Ser.No. 10/915,118 filed on Aug. 10, 2004 (U.S. Pat. No. 7,327,914); both ofwhich are fully incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to optical systems, and moreparticularly to adaptive signal processing with multimode opticalfibers.

BACKGROUND

Multimode optical fiber is a ubiquitous medium that is installed in avariety of applications, such as with universities, schools, hospitals,businesses and factories. In local and campus area networks, multimodefiber (MMF) has often been favored over single-mode fiber (SMF) becauseof the low cost of fiber installation and maintenance, and because ofthe much lower cost of transceiver components for MMF. In this context,“mode” generally refers to the characteristic of the propagation oflight (e.g., through a waveguide) that can be designated by a radiationpattern in a plane transverse to the direction of travel. “Single-modefiber (SMF)” thus has been used to generally refer to a fiber thatfacilitates light propagation that is designated by a single lightcharacteristic type (i.e., a single radiation pattern). “Multi-modefiber (MMF)” thus has been used to generally refer to a fiber thatfacilitates light propagation that is designated by two or more lightcharacteristic types (i.e., two or more radiation patterns).

From a practical standpoint, MMF has generally offered lower capacitythan SMF. Transmission rates in MMF are limited by the propagation ofmultiple transverse modes at different group velocities; this maytypically be referred to as modal dispersion, e.g., wherein a signal isspread in time due to different propagation velocities for differentmodes. SMF is typically free of this type of modal dispersion. Hence, inrecent decades, research on SMF systems has far outstripped work on MMFsystems. SMF systems can transport terabits per second over thousands ofkilometers. However, MMF systems have typically been limited to a bitrate-distance product well below 10 Gb/s-km.

In some aspects, wireless channel communications are analogous to MMFcommunications. Multipath fading in wireless systems was traditionallyviewed as a strictly deleterious phenomenon. Various techniques havebeen devised to overcome fading in single-input, single-output (SISO)links, including diversity and equalization, as well as multicarrier andspread spectrum modulation. In recent years, it has been realized thatmultipath fading actually creates additional spatial dimensions that canbe exploited by multi-input, multi-output (MIMO) techniques todramatically enhance wireless transmission capacity. The plurality ofmodes in MMF has traditionally been viewed as a strictly negative,bandwidth-limiting effect, and various techniques have been proposed tocounter modal dispersion.

Various approaches for eliminating modal dispersion in SISO MMF linkshave been proposed. For example, multimode fibers with substantially lowmodal dispersion have been developed. Wavelength-division multiplexing(WDM) can be used to increase the aggregate bit rate (but is relativelyhigh in cost). Various other techniques, including controlled launch,electrical equalization or subcarrier modulation can provide relativelylimited increase in bit rate-distance product associated with multimodefibers.

Among less conventional approaches, a segmented photodetector can beused to perform spatially resolved intensity detection. Thephotocurrents from the different segments can be processed usingdiversity combining and electrical equalization to mitigate the effectof modal dispersion. Another approach involves the use of diffractiveoptical elements to selectively excite one fiber mode in an attempt toreduce modal dispersion. With these approaches, fixed spatial filteringis used to launch into one “mode,” which is more precisely described asan eigenmode of an idealized round, straight fiber. In real fibers,random fabrication errors and bends lead to coupling between these idealeigenmodes over distances of centimeters to meters. Hence, even if onelaunches into a single ideal eigenmode, substantial modal dispersionstill occurs over transmission distances of practical interest, whichare tens to thousands of meters. Furthermore, in the presence of modalcoupling, slow changes in the fiber temperature and stress make modaldispersion time-varying, on a time scale typically of the order ofseconds. It is interesting to note that while modem graded-index fibershave far less modal dispersion than older step-index fibers, thevelocity matching in graded-index fibers actually enhances modecoupling, making it even more difficult to control modal dispersion bylaunching into one ideal eigenmode.

Over the years, several groups have proposed various approaches to MIMOtransmission in MMF. For instance, it has been suggested to exploitmultiple spatial degrees of freedom using angle multiplexing, i.e., bylaunching different information streams at different angles. While thisapproach is tantalizing and has been proposed by several groups over thepast two decades, it has failed because signals launched at differentangels become cross-coupled after propagating a few meters in step-indexor graded-index fibers.

SUMMARY

The present invention is directed to overcoming the above-mentionedchallenges and others related to the types of devices and applicationsdiscussed above and in other applications. These and other aspects ofthe present invention are exemplified in a number of illustratedimplementations and applications, some of which are shown in the figuresand characterized in the claims section that follows.

Various aspects of the present invention are applicable to a class ofadaptive spatial-domain signal processing approaches that mitigate theimpact of modal dispersion in SISO links. Other aspects of the presentinvention are applicable to approaches that facilitate MIMOtransmission, thus exploiting the multiple spatial dimensions in MMF andmultimode waveguides. In some instances, the MIMO approachessimultaneously address modal dispersion issues and achievemultiplicative capacity gains using spatial multiplexing.

According to an example embodiment of the present invention, adaptivespatial domain signal processing, responsive to a feedback signalindicative of data conveyed in the multimode waveguide, is used tomitigate interference in optical signals conveyed in a multimodewaveguide. In one implementation, intersymbol interference is mitigated.In another implementation, cross-channel interference is mitigated.

According to another example embodiment of the present invention, anoptical transmission system is adapted for transmitting light via amultimode optic medium. The system includes the multimode optic medium,a transmitter to transmit light via the multimode optic medium, and areceiver to receive light transmitted via the multimode optic medium.The system also includes a light processor arrangement, which isincluded as part of at least one of the transmitter and the receiver andwhich is responsive to a feedback signal indicative of data conveyed inthe multimode optic medium, to mitigate interference adaptively inoptical signals conveyed in the multimode optic medium.

The above summary is not intended to describe each illustratedembodiment or every implementation of the present invention. The figuresand detailed description that follow more particularly exemplify theseembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thedetailed description of various embodiments of the invention thatfollows in connection with the accompanying drawings, in which:

FIG. 1 shows a block diagram for a spatial-domain signal processingapproach involving a single-input, single-output (SISO) communicationsystem 100 using adaptive spatial filtering, according to anotherexample embodiment of the present invention;

FIG. 2 shows an example impulse response for the output RI_(out)(t)represented by the function “h(t)” as may be implemented in connectionwith one or more of the various embodiments herein, according to anotherexample embodiment of the present invention;

FIG. 3 shows an example response for the output RI_(out)(t) representedby the function “h(t)” as may be implemented in connection with one ormore of the various embodiments herein, according to another exampleembodiment of the present invention;

FIG. 4 shows a SISO communication system 400, according to anotherexample embodiment of the present invention;

FIG. 5A shows principal modes in multimode fiber of FIG. 5B, accordingto another example embodiment of the present invention;

FIG. 5B shows a fiber having an outer cladding surrounding an inner corethat transmits multimode optical signals, according to another exampleembodiment of the present invention; and

FIG. 5C shows principal modes in multimode fiber of FIG. 5B, accordingto another example embodiment of the present invention;

FIGS. 6A and 6B show alternatives for spatial multiplexing anddemultiplexing in M×M multi-input, multi-output (MIMO) links, accordingto other example embodiments of the present invention;

FIGS. 7A and 7B show an M×M MIMO communication approach using adaptivespatial filtering, according to another example embodiment of thepresent invention; and

FIG. 8 shows an adaptation algorithm system 800 for an M×M MIMO system,according to another example embodiment of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

The present invention is believed to be applicable to a variety ofdifferent types of devices and processes, and has been found to beparticularly suited for the transmission of light via fiber optic media.While the present invention is not necessarily limited to suchapplications, various aspects of the invention may be appreciatedthrough a discussion of examples using this context.

According to an example embodiment of the present invention, a multimodewaveguide, such as a multimode fiber (MMF), approach is adapted forexhibiting principal modes that propagate independently withwell-defined group delays in the presence (or absence) of modalcoupling. Adaptive signal processing is implemented (e.g., with spatiallight modulators (SLMs)) for the facilitation of the independentlypropagating principal modes. The transmission of light using theseapproaches is implemented using, e.g., intensity modulation with directdetection (IM/DD).

The MMF approach is adaptable for use with a variety of transmission andprocessing approaches. In one implementation, the MMF is implementedwith single-input, single-output (SISO) processing, withtransmitter-based spatial filtering used to excite a single principalmode. In another implementation, the MMF is implemented withmultiple-input, multiple-output (MIMO) processing, usingtransmitter-based and receiver-based spatial filtering to transmitmultiple information streams on different principal modes.

In another example embodiment of the present invention, a lighttransmission arrangement and approach involves spatially filtering lightthat is transmitted via a MMF medium. Light from a source such as alaser is intensity-modulated using one or more of a variety ofapproaches. The intensity-modulated light (signal) is spatiallyFourier-transformed via a lens, spatially filtered with an SLM,spatially inverse-Fourier-transformed and coupled to the MMF medium. Thelight signal is then passed via the MMF medium to a light detector(e.g., a photodetector) where data is gleamed from the light.

In one implementation, on-off keying (OOK) is used for intensitymodulation and typically involves characterizing data by the presence orabsence of light. In this regard, light turned “ON” is used tocharacterize a first logic state (e.g., a “1” bit) and light turned“OFF” is used to characterize another logic state (e.g., a “0” bit).

In another implementation, multilevel pulse-amplitude modulation is usedfor intensity modulation in spatially filtering the light. Multilevelpulse-amplitude modulation involves, e.g., using the amplitude ofindividual, regularly spaced pulses in a pulse train is varied inaccordance with some characteristic of the modulating signal. Thisvariation is used to convey information.

In another embodiment, a feedback loop between the light detector andthe light intensity-modulating function is used to reduce intersymbolinterference (ISI), which is typical of an overlap between signals, orpulses, in optical communications. The ISI received in each symbol(optical information) is estimated, and estimates from each symbol areaccumulated to yield a cost function representing the ISI (e.g., asdescribed below, to yield a cost function that is the minimum noise-freeeye opening (d_(min))). The ISI cost function is fed back to thetransmitter, where an adaptive algorithm computes updates to the SLMpixel settings so as to reduce ISI. In some instances, the feedback bitrate is typically negligible in comparison to the transmission rate.

As may be applicable in connection with certain example embodiments, theterm “interference” may generally apply to deleterious light signalsthat are related to transmitted light signals. Examples of suchinterference include intersymbol interference and cross-channelinterference caused by modal dispersion and modal coupling in multimodewaveguides.

As used with various example embodiments and implementations herein,principal modes (PMs) are a generalization of the principal states ofpolarization (PSPs) for light propagation in optical fibers, and whichare often referred to as polarization-mode dispersion (PMD). Consideringthe spatial dependence of linearly polarized (LP) modes in MMF, thepolarization degree of freedom can be ignored for analysis purposes asused with various embodiments herein (i.e., PMD is generally weaker thanmodal dispersion in most MMFs).

As an example representation of PMs that may be used in connection withvarious example embodiments herein, one implementation involves anarrowband optical signal that is centered at frequency ω and propagatesin a non-ideal fiber that supports N propagating modes. An electricfield pattern |e_(a)

at the fiber input propagates to a field pattern |e_(b)

at the fiber output as represented by

|e _(b)

=T(ω)|e _(a)

  (1)

T(ω) is an N×N matrix representing propagation (both loss and phasechange) as well as modal coupling. Neglecting mode-dependent loss (areasonable first-order approximation for lower-order modes), T(ω) isrepresented as

T(ω)=e ^(γ(ω)) U(ω),   (2)

where γ(ω) is a scalar representing loss and phase change (averaged overthe N modes) and U(ω) is an N×N unitary matrix representing losslesspropagation and modal coupling.

In an N-mode fiber, there exists a set of input PMs |a_(n)

, n=1, . . . , N, and a set of corresponding output PMs |b_(n)

, n=1, . . . , N. An input PM |a_(n)

is launched, with the corresponding output PM |b_(n)

appearing at the output generally without cross-coupling to any of theother PMs:

|b _(n)

=T(ω)|a _(n)

  (3)

By analogy with PSPs, the PMs are defined such that, given an input PM|a_(n)

, the output PM |b_(n)

is independent of frequency ω to a first order. Thus, a pulse-modulatedoptical signal transmitted with modal shape |a_(n)

retains its integrity and is received in modal shape |b_(n)

and generally without modal dispersion to a first order in ω. The groupdelay operator can be defined as follows:

$\begin{matrix}{{{F(\omega)} = {{- i}\; {U^{+}(\omega)}\frac{\partial U}{\partial\omega}}},} & (4)\end{matrix}$

and can show that each input PM is an eigenmode of F(ω):

F(ω)|a _(n)

=τ_(n) |a _(n)

,   (5)

where the eigenvalue τ_(n) is simply the group delay of the PM. The Ninput PMs (or output PMs) are a complete, orthonormal set for describingpropagating fields in the MMF. In the absence of modal coupling, the PMsare very nearly identical to the ideal fiber modes.

The above-described properties make the PMs suitable for SISOtransmission with minimal modal dispersion, or for MIMO transmissionwith minimal cross-coupling and modal dispersion. Since the PMs dependon the matrix U(ω), which includes lossless propagation and modalcoupling, the PMs will evolve slowly as stress, temperature and otherfiber perturbations change. In the communication schemes described inconnection with various example embodiments herein, it is not necessaryto know what the PMs are at any point in time. When PMs are known toexist, spatial signal processing techniques can be adapted as the PMschange in order to optimize data transmission as described furtherbelow.

FIG. 1 shows a block diagram for a spatial-domain signal processingapproach involving a single-input, single-output (SISO) communicationsystem 100 using adaptive spatial filtering, according to anotherexample embodiment of the present invention. The MMF supports a multiplenumber (N) of principal modes (PMs), and hence, can be described as anN×N MIMO system with inputs and outputs given by the |a_(n)

and the |b_(n)

, respectively. A scalar input intensity signal I_(in)(t) is mapped ontoPMs including modes 112, 114, 116 and 118 (or more, depending on thetotal number of modes N), by a spatial light modulator (SLM) 110. TheSLM 110 is a 1×N SIMO (single-input, multiple-output) system.

The PMs 112-118 are passed from the SLM 110 to an N-mode fiber 120having N principal input modes (hereinafter “inputs”) and N principaloutput modes (hereinafter “outputs”). The fiber 120 couples the outputof the SLM 110 to a receiver photodetector 130 having N inputs and asingle output. The PMs are generally orthogonal to the output plane ofthe fiber 120; thus, the total intensity is the sum of the squaredintensities from each of the PMs (as represented by functions 142, 144,146 and 148). The photodetector 130 (e.g., a square-law photodetector)detects the intensities in each of the N orthogonal PMs by squaring thefield magnitude of each PM, as represented by shown squaring functions142, 144, 146 and 148, to obtain the intensity. Each function physicallycoincides within the photodetector but is mathematically processed asshown. A summation block function 132 sums the detected intensities(integrating each intensity over the output plane of the fiber 120) toyield a scalar photocurrent output RI_(out)(t). In this regard, thephotodetector 130 is implemented here as an N×1 MISO (multiple-input,single-output) system.

FIG. 2 shows an example impulse response for the output RI_(out)(t)represented by the function “h(t).” In this example, the SLM is notadapted for reducing ISI, such that h(t) includes contributions fromeach of the N PMs. In this regard, h(t) exhibits modal dispersion asshown. The plot of the function h(t) is thus asymmetrical (though notnecessarily so), representing varied characteristics between the PMs andtheir respective contributions.

FIG. 3 shows an example response for the output RI_(out)(t) representedby the function “h(t).” In this example, the SLM is adapted for reducingISI (e.g., using a feedback loop from a receiver), where h(t) includesthe contribution from only one PM (or from several PMs having nearlyidentical group delays). In this regard, the function h(t) does notexhibit the modal dispersion as shown in FIG. 2 and, thus, the resultingplot exhibits an impulse that is generally narrow (short) in time (i.e.,the pulse width is less than one symbol interval). The description belowin connection with FIG. 4 further discusses an adaptive approach forreducing ISI.

In one implementation, the SISO system impulse response h(t) shown inFIG. 2 and FIG. 3 is computed as follows. The principal mode |a_(n)

is let to correspond to a spatial electric field pattern a_(n)(x, y) inthe MMF input plane, and A_(n)(k_(x), k_(y)) is let to denote itsspatial Fourier transform in the SLM plane. The complex reflectance ofthe SLM is denoted by V(k_(x), k_(y)). Considering only first-ordermodal dispersion, the impulse response is then given by:

$\begin{matrix}{{h(t)} = {^{{- \alpha}\; L}{\sum\limits_{n = 1}^{N}{{{\int{\int{{V^{*}\left( {k_{x},k_{y}} \right)}{A_{n}\left( {k_{x},k_{y}} \right)}{k_{x}}{k_{y}}}}}}^{2}{{\delta \left( {t - \tau_{n}} \right)}.}}}}} & (6)\end{matrix}$

The factor of e^(−aL) represents fiber loss. Each PM contributes, to theimpulse response, an impulse delayed by the corresponding group delayand scaled by the absolute square of the inner product between the SLMreflectance and the spatial Fourier transform of the PM mode fieldpattern. When chromatic dispersion and higher-order modal dispersion areincluded, each impulse is broadened into a pulse of finite width. If theSLM is discretized into square pixels of size Δk×Δk at locations(k_(xi), k_(yi)), the impulse response is approximated by:

$\begin{matrix}{{{h(t)} \approx {^{{- \alpha}\; L}\Delta \; k^{2}{\sum\limits_{n = 1}^{N}{{{\sum\limits_{i,j}{{V^{*}\left( {i,j} \right)}{A_{n}\left( {i,j} \right)}}}}^{2}{\delta \left( {t - \tau_{n}} \right)}}}}},} & (7)\end{matrix}$

where (i, j) is shorthand for (k_(xi), k_(yj)).

Equation (7) above shows the impulse response of a SISO link controlledby optimizing the SLM reflectance on a finite set of pixels {i,j}. Insome instances, the pixel reflectance values are quantized to a smalldiscrete set of phases (e.g., binary phase,V(i,j)ε{e^(iπ),e^(i0)}={−1,1}) or amplitudes (e.g., binary amplitudeV(i,j)ε{0,1}). Various example approaches for the implementation ofEquation (7) for SISO adaptive algorithms are described herein. In someinstances, modal dispersion is controlled using about 400 binaryphase-only pixels.

FIG. 4 shows a SISO communication system 400, according to anotherexample embodiment of the present invention. The system 400 may beimplemented using an approach that is similar, for example, to thatimplemented with the system discussed in connection with FIGS. 1-3. Thesystem includes a transmitter 410 adapted to transmit opticalinformation via a multimode (MMF) fiber 420 to a receiver 430. Alow-rate feedback channel 440 provides feedback from the receiver 430 tothe transmitter 410.

The transmitter 410 includes an adaptive algorithm function 412, aspatial light modulator (SLM) 414, a Fourier lens 416 and an on-offkeying (OOK) modulator 418. Light from a laser is intensity modulatedusing the OOK modulator 418, with the output of the intensity-modulatedlaser light being represented by a modulated signal, I_(in)(t). In someimplementations, the OOK modulator 416 is replaced with a multilevelpulse-amplitude modulator that modulates the intensity of the laserlight. The adaptive algorithm function 412 uses feedback from thereceiver 430 to adjust the SLM 414 to reduce ISI. Using the adaptivealgorithm function 412, the modulated signal I_(in)(t) is spatiallyFourier-transformed with the Fourier lens 416, spatially filtered withthe SLM 414, spatially inverse Fourier-transformed, and coupled into theMMF 420.

The receiver 430 includes a photodetector 432, a clock and data recoveryfunction 434 and an ISI estimation function 436. A signal I_(out) (t) isoutput from the MMF 420 and photodetected at photodetector 432. Theclock and data recovery function 434 processes the output of thephotodetector for use by the ISI estimation circuit 436 and forgenerating a data output (for recording or otherwise processing datareceived via the MMF 420). The ISI received in each symbol is estimatedat the ISI estimation function 436 and the estimates are accumulated toyield an ISI cost function representing (e.g., d_(min), the minimumnoise-free eye opening). The ISI cost function is output from the ISIestimation function 436 and fed back to the transmitter 410 via thelow-rate feedback channel 440, where the adaptive algorithm functioncomputes updates to pixel settings of the SLM 414.

FIGS. 5A-5C show principal modes in multimode fiber, with the fibershown in FIG. 5B and FIGS. 5A and 5C respectively showing two of N PMsin the fiber, according to another example embodiment of the presentinvention. The fiber in FIG. 5B has an outer cladding 500 surrounding aninner core 510 that transmits multimode optical signals (i.e., the fiberis an N mode MMF). The PMs shown in FIGS. 5A and 5C depict evolution ofspatial field patterns and pulse intensities for the fiber in FIG. 5B.Each principal mode may be in one or two polarizations, and is a linearcombination of up to N idealized fiber modes. For simplicity, thisfigure assumes that each principal mode is a combination of only twomodes; however, multiple modes can be combined and implemented usingsimilar approaches for other example embodiments.

FIG. 5A and FIG. 5C show two PMs, I_(k)(t) and I_(l)(t), which are apair of mutually orthogonal spatial field patterns; accordingly, thecorresponding output PMs represent orthogonal field patterns. A pulselaunched into one of the input PMs splits apart into two pulses as itpropagates, but arrives at the output as a single pulse in thecorresponding output PM. In the general case where each PM includes acombination of up to N modes, a pulse launched in an input PM splitsapart into as many as N pulses but arrives as a single pulse in thecorresponding output PM.

FIGS. 6A and 6B show alternatives for spatial multiplexing anddemultiplexing in M×M multi-input, multi-output (MIMO) links, accordingto other example embodiments of the present invention. The case M=2 isillustrated for simplicity. FIG. 6A shows multiplexed operation ofspatial light modulators and FIG. 6B shows the combining and splittingof signals using beamsplitters (BS). The beamsplitter loss is 1/M ateach of the transmitter and receiver. In the case M=2, the transmitter(but not receiver in general) may use a polarizing beamsplitter, whichis lossless in principle. In an M×M MIMO system (e.g., as shown in FIGS.6A and 6B), the transmitter spatially multiplexes M independentinformation streams by launching them into different input PMs. Thereceiver spatially demultiplexes these M streams by coupling them fromthe corresponding output PMs onto separate photodetectors.

In FIG. 6A, a single lens and SLM combination (620, 630) is used at eachend of a link including MMF 610. The transmitter SLM 620 can bedescribed as a multiplex of M different holograms, each of which mapsone of the input signals to a different input principal mode. Thereceiver SLM 630 can be described similarly as the multiplex of Mdifferent holograms. Each SLM may be implemented to incur, for example,a power loss of up to 1/M due to multiplexing and may incur crosstalk.The multiplexing loss and crosstalk level may depend on a number offactors, including M (the number of inputs and outputs), N (the numberof PMs), the number of SLM pixels, the number of quantization levels ofthe SLM reflectance, and differences in the spatial field patterns ofthe various PMs chosen for spatial multiplexing.

In an alternate spatial multiplexing configuration, M separate SLMs areused to encode the transmitted signals into different PMs, and the Mbeams are combined using M−1 beamsplitters. Likewise, the receiver usesM−1 beamsplitters and M separate SLMs to perform spatial demultiplexing.As shown in FIG. 6B, for small M, it is feasible to simply use Mnon-overlapping regions of a single SLM at each end of the link 611,with SLM arrangements (with beamsplitters) 621 and 622 at input andoutput positions respectively. For example, a commercially available256×256 pixel SLM can multiplex or demultiplex several signals, assumingeach signal requires several hundred to several thousand SLM pixels.Using ordinary beamsplitters, multiplexing and demultiplexing losses of1/Mare incurred. In some implementations, the multiplexing loss can bereduced by a factor of two by using a polarizing beamsplitter.

FIGS. 7A and 7B show an M×M MIMO communication approach using adaptivespatial filtering, according to another example embodiment of thepresent invention. The case M=2 is illustrated for simplicity. FIG. 7Ashows a physical configuration of the communications approach with anM×M MIMO transmission link, based on multiplexed operation of spatiallight modulators at transmitter and receiver arrangements 720 and 730respectively at the transmitter and receiver ends of a MMF 710. Mindependent intensity-modulated data signals are transmitted andreceived by direct-detection receivers. From the received signals, ISIand cross-channel interference (XCI) are estimated, and estimates areaccumulated to yield cost functions representing ISI and XCI. Algorithmsrun at both the transmitter and the receiver arrangements 720 and 730update the SLM pixels in order to optimize the cost functions. Thetransmitter and receiver arrangements 720 and 730 may be implemented,for example, in a manner similar to that discussed with FIG. 4 andimplemented for similar components (with an SLM at the receiver end).

When used for MIMO systems, at the transmitter end of a lighttransmission arrangement (e.g., transmitter arrangement 720 of FIG. 7A),the polarization sensitivity of SLMs are accommodated withoutnecessarily modifying the spatial multiplexing schemes. At the receiverend (e.g., receiver arrangement 730 of FIG. 7A), apolarization-diversity spatial demultiplexer with a polarizingbeamsplitter and a pair of SLMs is used. Each spatially demultiplexedlight channel is detected by a pair of receivers in two orthogonalpolarizations, and the corresponding outputs are combined.

FIG. 7B shows a block diagram depicting spatial-domain signalprocessing, functionally representing an approach similar to that shownin FIG. 7A. The transmitter arrangement 720 corresponds to transmitterarrangement 721, the MMF 710 corresponds to MMF 711 and the receiverarrangement 730 corresponds to receiver arrangement 731. As explainedabove, the MMF 711 can be described as an N×N MIMO system with inputsand outputs given by the |a_(n)

and the |b_(n)

, respectively. At the transmitter arrangement 721, the vector of Minput signals {I_(1,in)(t), . . . , I_(M,in)(t)} is mapped onto theprincipal modes by the SLM, which is an M×N MIMO system. Similarly, theSLM at the receiver arrangement 731 can be viewed as an N×M MIMO system.

Various types of adaptive algorithms can be used to aid in theadjustment of an SLM in order to reduce ISI in SISO links, or in theadjustment of one or more SLMs to reduce ISI and XCI in MIMO links. Invarious implementations, a low-complexity single-coordinate ascent (SCA)scheme is used for light processing involving the generation of feedbackfor use in reducing ISI (e.g., as can be implemented with FIG. 4). Insome implementations, the SCA scheme is used for also reducing XCI ascan be implemented, e.g., with FIG. 7A. In the reduction of ISI, asingle SLM pixel is changed, and the ISI is measured for a number ofbits in order to form an accurate estimate of the cost function d_(min).The cost function estimate is fed back to the transmitter, where it isdecided whether or not to keep the last pixel change made. The SCAtechnique can adapt from an arbitrary starting point without priorknowledge of the MMF. This SCA scheme works as well as much morecomplicated algorithms based on semidefinite programming. For moreinformation regarding light processing in general, and for specificinformation regarding light processing approaches involving the costfunction d_(min) and/or SCA schemes, reference may be made to U.S. Pat.No. 7,194,155, entitled “Adaptive Control for Mitigating Interference ina Multimode Transmission Medium”, which is fully incorporated herein byreference.

In another implementation, a 10 Gb/s SISO link transmitting over a 1 kmlong, 64 μm step-index fiber, which supports 88 modes (per polarization)at 850 nm wavelength is implemented with light transmission. A binaryphase-only SLM (V(i,j)ε{e^(iπ),e^(i0)}={−1,1}) is used.

FIG. 8 shows an adaptation algorithm system 800 for SISO transmission,according to another example embodiment of the present invention.Multiple copies of the system 800 can be implemented for MIMO systems.The system 800 is implemented, for example, with light transmission asshown in one or more example embodiments herein (e.g., with block 412 ofFIG. 4 and/or with the transmitter/receiver arrangements 720 and 730 ofFIG. 7A). Abbreviations shown in and used with FIG. 8 and theircorresponding meanings include the following:

TIA: transimpedance amplifier,

CDR: clock and data recovery,

DAC: digital/analog converter,

ISI: intersymbol interference,

FPGA: field-programmable gate array, and

SLM: spatial light modulator.

While the present invention has been described with reference to severalparticular example embodiments, those skilled in the art will recognizethat many changes may be made thereto without departing from the spiritand scope of the present invention.

1. For use with a multimode waveguide, a method comprising: for datacommunication via the multimode waveguide, passing light signals througha transmitter having a light-signal processor; using the light-signalprocessor to implement an adaptive spatial-domain signal process thatresponds to a feedback signal indicative of data conveyed in themultimode waveguide; and using the feedback signal and the light-signalprocessor, manipulating the light signals to mitigate interference inoptical signals conveyed in the multimode waveguide.
 2. The method ofclaim 1, wherein manipulating the light signals to mitigate interferenceincludes mitigating modal dispersion.
 3. The method of claim 1, furtherincluding the step of intensity modulating light for transmission in themultimode waveguide based on the feedback signal.
 4. The method of claim3, wherein intensity modulating light includes using on-off keying. 5.The method of claim 1, further including the step of mitigatingintersymbol interference responsive to the feedback signal.
 6. Themethod of claim 1, further including the step of spatially filteringlight conveyed in the multimode waveguide based on the feedback signal.7. The method of claim 1, further including the step of conveying lightsignals in the multimode waveguide using a single-input, single-outputlink with the multimode waveguide.
 8. The method of claim 1, furtherincluding the step of conveying light signals in the multimode waveguideusing a multiple-input, multiple-output link with the multimodewaveguide.
 9. The method of claim 9, further including the step of usingan adaptive spatial domain signal process to mitigate cross-channelinterference between different information streams passing in themultimode waveguide.
 10. The method of claim 1, further including thestep of spatially Fourier-transforming a form of the light signals usinga Fourier lens.
 11. The method of claim 1, further including the step ofspatially filtering optical signals at a transmitter based on thefeedback signal and conveying the spatially filtered optical signals inthe multimode waveguide.
 12. The method of claim 1, further includingthe step of spatially filtering optical signals at a receiver coupled toreceive the optical signals conveyed in the multimode waveguide.
 13. Themethod of claim 1, further including the step of using adaptive spatialdomain signal processing to mitigate modal dispersion in optical signalsconveyed in the multimode waveguide.
 14. The method of claim 1, furtherincluding the step of estimating intersymbol interference, generatingthe feedback signal therefrom, and using the feedback signal to mitigateintersymbol interference in subsequently passed optical signals.
 15. Anapparatus for communicating light, the system comprising: a multimodewaveguide means for carrying light signals; and adaptive spatial domainsignal processing means for mitigating modal dispersion in opticalsignals conveyed in the multimode waveguide in response to a feedbacksignal indicative of data conveyed in the multimode waveguide.
 16. Asystem for transmitting light, the system comprising: a multimode opticmedium; a transmitter to transmit light via the multimode optic medium;a receiver to receive light transmitted via the multimode optic medium;and a light processor arrangement, included as part of at least one ofthe transmitter and the receiver and responsive to a feedback signalindicative of data conveyed in the multimode optic medium, to mitigateinterference adaptively in optical signals conveyed in the multimodeoptic medium.
 17. The system of claim 16, wherein the transmitterincludes the light processor arrangement, and wherein a low-ratefeedback channel provides the feedback signal from the receiver to thetransmitter.
 18. The system of claim 16, wherein the transmitterincludes the light processor arrangement, and wherein the receiver alsoincludes a light processor arrangement configured and arranged tomitigate interference in optical signals conveyed in the multimode opticmedium.
 19. The system of claim 16, further including a separatefeedback channel for communicating the feedback signal from the receiverto the transmitter.
 20. The system of claim 16, further including aseparate feedback channel configured to carry the feedback signal fromthe receiver to the transmitter, wherein the transmitter includes aspatial light modulator and a lens, and wherein the separate feedbackchannel is used to adjust the spatial light modulator to reduceintersymbol interference in subsequently passed optical signals.
 21. Thesystem of claim 16, wherein the light processor arrangement isconfigured and arranged to implement adaptive spatial domain signalprocessing to mitigate cross-channel interference between differentinformation streams passing in the multimode optic medium.
 22. Thesystem of claim 16, wherein the transmitter is configured to intensitymodulate light for transmission in the multimode optic medium based onthe feedback signal.
 23. The system of claim 22, wherein the transmitteris configured to use on-off keying to intensity modulate the light. 23.The system of claim 16, wherein the receiver is adapted to estimateintersymbol interference and use the estimate to generate the feedbacksignal, and wherein the receiver provides the feedback signal to thetransmitter and the transmitter uses the feedback signal to mitigateintersymbol interference in subsequently passed optical signals.
 24. Thesystem of claim 23, wherein the receiver is adapted to estimateintersymbol interference and use the estimate to generate the feedbacksignal, and wherein the receiver provides the feedback signal to thetransmitter and the transmitter uses the feedback signal to mitigateintersymbol interference in subsequently passed optical signals, andwherein the receiver is adapted to estimate the intersymbol interferenceby generating a cost function.
 25. The system of claim 23, wherein thereceiver is adapted to estimate intersymbol interference and use theestimate to generate the feedback signal, and wherein the receiverprovides the feedback signal to the transmitter and the transmitter usesthe feedback signal to mitigate intersymbol interference in subsequentlypassed optical signals, and wherein the receiver is adapted to estimatethe intersymbol interference by estimating intersymbol interference fora plurality of symbols and accumulating the estimated interference fromthe symbols to generate a cost function.